258 



PHYLUM ECHINODEEMATA. 



DORSAL 



Left 



Right 



RM 



RH 



anterior part of intestine is attached by a dorso median (adult) 

 mesentery (Fig. 181, 26) to the body wall in the dorsal inter- 

 radius (Fig. 182, J/J. At the first bend the mesentery passes 

 across the left dorsal radius to the left dorsal interradius, 

 where it is attached all along the second or forward reach of the 

 intestine (M 2 }. At the second bend the mesentery passes 

 across the intervening radii and interradii into the right ventral 

 interradius. where it is attached all along the third backwardly 

 directed reach of the intestine (M 3 ). In Synapta the alimentary 



canal is straight, but as is 

 shown by the attachment of 

 the mesentery it has the same 

 spiral course round the body 

 wall as that just described. 

 The anus, which is typically 

 terminal, may, like the mouth, 

 be apparently shifted on to 

 the dorsal or ventral surface. 



The wall of the gut consists of 



(1) an internal epithelium which 

 lias a cuticle and may be ciliated, 



(2) a layer of connective tissue con- 

 taining blood spaces, (3) a muscular 

 layer consisting of longitudinal and 

 circular fibres, (4) an outer connec- 

 tive tissue layer, and (5) the peri- 

 toneal epithelium. 



The respiratory trees (Fig. 

 181) are two hollow much- 

 branched structures placed 

 right and left in the body 

 cavity and opening together or 

 by separate openings into the cloaca. The ultimate branches 

 of the organ end in ampulla-like dilatations which may also te 

 found along the course of the branches themselves. 



The walls consist of (1) an inner, probably ciliated, epithelium often 

 in more than one layer ; projections into the cavity, caused by cells con- 

 taining yellow pigment granules, are present ; (2) a connective tissue 

 layer, (3) a muscular layer, (4) an outer connective tissue layer, and (5) 

 a layer of peritoneal epithelium. The respiratory trees do not communi- 

 cate with the body-cavity, and they are absent in the Synaptidae and 

 Elasipodidae, but in the latter group there is a forwardly directed caecum 



VENTRAL 



FIG. 182. Diagrammatic transverse section 

 through a Holothurian seen from the 

 aboralpole, to show the course of the gut, 

 the attachment of the mesentery and the 

 enumeration of the radii adopted in the 

 text. The vertical dotted line gives the 

 position of the median plane, and the x 

 the position of the transversely cut verti- 

 cal axis. 1, 2, 3 represent" the three 

 stretches of the alimentary canal, and M. 

 3/2, 3/3 the mesenteries attaching them. 

 RI-RV mark the radii numbered, and 

 IRl-IRs the interradii. //fo is the dorsal 

 interradius of the bivium, which contains 

 the water-pore and generative opening. 



